WATCH: Mourners lit candles and laid flowers and toys on the banks of the Volga River today after dozens were killed in the sinking of a crowded tourist boat. The confirmed death toll has risen to 72 but many more are feared dead. (Video by RFE/RL's Tatar-Bashkir Service)

Russian authorities have opened a criminal investigation into the sinking of a tourist boat on the Volga River, as the country observed a day of mourning for the more than 100 victims of the tragedy.

The crowded boat sank in stormy weather on July 10 while carrying at least 200 people, far exceeding its capacity. Eighty people were rescued.

The death toll has risen to at least 116, as divers searching the boat discovered some 50 bodies, most of them children, in the ship's recreation room.

Russia's Investigative Committee said today a tour agency director and safety official had been arrested and that a criminal case has been opened against them.

A separate case was opened against the captains of two ships that reportedly passed by at the time of the sinking but did not stop to offer help.

A spokeswoman for the Prosecutor-General's Office said the ship was overloaded, had no licence to carry passengers, and had a problem with its left engine when it sailed out in a storm. State TV said it had never undergone major repairs. The boat was built in 1955 in then communist Czechoslovakia.

Russian President Dmitry Medvedev said on July 11 that the ship would not have sunk if safety rules had been observed.

"It is clear already today that this accident would not have happened, even in such adverse weather conditions, if all the safety and technical regulations had been followed," the president said.

"It is necessary to thoroughly investigate why this happened and why the boat owner was using the vessel in this technical condition."